In general with air conditioning equipment, a refrigerant sensor is installed in a closed circuit in which refrigerant is circulated, to detect if the amount of refrigerant in the closed circuit is sufficient or insufficient. The present inventors have proposed a refrigerant sensor, as disclosed in pending Japanese Patent Application No. 3-357552 (referred to as prior art in the following) which comprises a sensor body having a refrigerant flow path portion for the passage of refrigerant, provided in a flow path between a condenser and evaporator in a cooling cycle, a refrigerant chamber located in the sensor body above the refrigerant flow path portion and communicated with the interior of the refrigerant flow path portion, and a thermo-sensitive device provided inside the refrigerant chamber for detecting if refrigerant inside the chamber is in a liquid phase condition.
The refrigerant sensor constructed as above is fitted to piping between a receiver tank which holds the liquid phase refrigerant condensed by the airconditioning equipment condenser, and an expansion valve whereby liquid phase refrigerant is expanded and evaporated. With drive to the air conditioning equipment, the compressor comes into operation, and if the refrigerant circulating in the cooling cycle is filled to a sufficient amount, the interior of the refrigerant flow path portion of the refrigerant sensor is filled with the liquid phase refrigerant flowing there through until the surface of the liquid reaches the upper surface of the refrigerant chamber interior.
At this time, since the thermo-sensitive device inside the refrigerant chamber is immersed in the liquid phase refrigerant, there is a significant cooling effect due to the refrigerant, so that the resistance value of components such as a thermistor which make tip the thermo-sensitive device is maintained at a low temperature value. However, if the amount of refrigerant in the cooling cycle is diminished due to refrigerant leakage etc., the liquid level of refrigerant inside the refrigerant chamber is lowered and the thermo-sensitive device exposed above the refrigerant liquid surface. Since the cooling effect of gaseous phase refrigerant is less than that of liquid phase refrigerant, the temperature of the thermo-sensitive device rises, with a consequent change in resistance, causing a change in the current flowing through a warning device, thereby bringing on a light notifying of the refrigerant deficiency.
With the refrigerant sensor according to the above prior art, upward or downward movement of the liquid surface of the refrigerant in the refrigerant chamber is possible even with no change in refrigerant quantity, due to fluctuations in the pressure of refrigerant in the piping caused by a change in compressor rotational speed (with an automotive compressor, due to a change in rotational speed of the engine driving the compressor). As a result, the level of the liquid surface of the refrigerant becomes greater than or less than that of the thermo-sensitive device, immersing or exposing the device, with a consequent change in cooling effect.
Hence, when a refrigerant deficiency is detected with a refrigerant sensor which uses for example a thermistor having a higher resistance at lower temperatures for the thermo-sensitive device, then the characteristics as shown in FIG. 6 are obtained. In this figure, the compressor rotational speed is represented by tile horizontal axis, tile current I in the warning device circuit is represented by the vertical axis, and tile characteristic curves B10, B20, B40, B60, B80, B100, and B120 show the characteristics for respective refrigerant fill percentages ranging from 10, 20, 40, 60, 80, 100 to 120% with 100% being the ideal. The 10 current value in FIG. 6 is the value for the case when the warning device activates a tamp or the like to give a refrigerant insufficiency warning.
With such a refrigerant sensor, even if there is a sufficient fill amount inside the cooling cycle 1, with a refrigerant F fill percentage of 60% for example as shown by tile characteristic line B60, then if the compressor rotational speed N is less than approximately 2000 rpm, a refrigerant insufficiency is reported due to an insufficiency condition. However, by increasing the rotational speed, a sufficient condition is erroneously sensed and the refrigerant insufficiency warning is canceled. Moreover, for the case at the extreme end of the graph with the low refrigerant fill percentage characteristic line B10, if the compressor rotational speed N is above approximately 5000 rpm, then in spite of the fact that a refrigerant insufficiency condition exists, it is not possible to accurately judge if a sufficient or insufficient condition exists.
In view of the above mentioned problems with the refrigerant insufficiency detector for airconditioning apparatus, it is an object of the present invention to provide a refrigerant insufficiency detection method and apparatus for an airconditioning apparatus which can accurately detect an insufficiency condition in the fill amount of refrigerant irrespective of compressor rotational speed.
It is a further object of the present invention to accurately detect a refrigerant insufficiency so as to reliably prevent unloaded operation of the compressor, and prevent problems such as seizing of the compressor.